1. Field of the Invention
The invention comprises a method for cleaning of heat exchangers after passage through the heat exchangers of a fluid which comprises vegetable dry matter, and which by transport through a passage will tend to generate deposits on exposed positions in the passage. Such fluid is typically a fluid, pulpy material, preferably fruit or vegetable juice, e.g. orange, pineapple, grape fruit, banana, grape or apple juice, or carrot, tomato, or celery juice. For the sake of brevity in the following such fluid will be identified as "fluid, pulpy material", and the deposits will be identified as pulp or pulp particles. The heat exchangers are preferably of the plate type.
In order to generate a good heat exchange the plates in a plate heat exchanger are usually placed in close proximity to each other, e.g. with a gap of around 2.2-6 mm, preferably 3.5-4 mm. Also, two adjacent plates have to touch each other at several hundred pointed positions per m.sup.2 in order to secure the physical stability of the plate package during operation, when the pressure differential is at its maximum. It has been found, however, that the free passages between two adjacent heat exchanger plates are being clogged or at least narrowed considerably after passage of fluid, pulpy material, due to obstinate adherence of pulp particles to the plates, especially at the above indicated pointed positions, where two adjacent plates touch each other. This layer of pulp particles has to be removed from the heat exchangers in order to reestablish a satisfactory performance of the heat exchangers and furthermore for hygienic reasons. Also, if the pulp particles are not removed effectively, they can be released during a later heat exchange with a clear liquid, e.g. clear apple juice, which would be detrimental to the quality of the clear liquid. Hitherto two methods have been used for the purpose of removing the pulp particles from the heat exchangers. In the first place the plate heat exchanger assembly can be dismantled completely and cleaned mechanically. This is effective, but also a time consuming and cumbersome method. Ordinarily the so-called cip method (cip is short for cleaning in place) is used, i.e. great amounts of high pressure water is flushed through the heat exchanger in order to remove the layer of pulp particles, and also hot lye (NaOH) and/or a peroxide solution or hot lye and complex builders, such as EDTA, is flushed through for cleaning purposes. Even if this method is effective, it is time consuming, as it ordinarily lasts around 2-4 hours, and also energy consuming due to the necessity of the supply of high pressure water, the lye and/or peroxide solution treatment damages the rubber gaskets, which have to be replaced at shorter intervals, and also the lye and/or peroxide treatment presents an environmental problem. Furthermore, especially EDTA also presents an environmental problem. Also, in case the cip method is used, it has been found that even so it is necessary at certain time intervals, e.g. once a week, to dismantle the plate heat exchange assembly completely and carry out a mechanical cleaning. This cleaning problem is not a minor problem, but a problem which haunts the industrial world to a significant degree in consideration of the fact that on a global basis the annual amount of fluid, pulpy material which is sent through heat exchangers, run into millions of tons. Great efforts to solve this problem have been exercised, but so far in vain. An attempt to circumvent the problem is the so-called free flow heat exchanger, in which the gap between adjacent plates is much larger than in ordinary plate heat exchangers and in which no contact points between adjacent plates exist; however, the heat exchange capacity in such free flow heat exchangers is unsatisfactorily low.
Thus, the purpose of the invention is the provision of a method for cleaning of heat exchangers after passage of fluid, pulpy material, which is fruit or vegetable juice, through the heat exchangers, which method should be less time and energy consuming and present fewer environmental problems than the hitherto used methods of this kind.
Now, surprisingly, it has been found, according to the invention that the purpose of the invention can be fulfilled enzymatically.
Thus, the method according to the invention for cleaning of heat exchangers after passage through the heat exchangers of a fluid, which comprises vegetable dry matter, and which by transport through a passage will tend to generate deposits on exposed positions in the passage is characterized by the fact that the heat exchangers are exposed to a cip treatment with a solution of an enzyme preparation, which contains pectolytic, cellulolytic, and hemicellulolytic activities.
Surprisingly it has been found that the indicated enzymatic cip treatment effectively cleans the heat exchanger plates in much less time than usual, e.g. in around an hour, that high pressure treatment with large amounts of water is unnecessary, and that the lye or peroxide solution treatment is superfluous after the enzymatic cip treatment, whereby environmetal problems due to the lye or peroxide solution treatment are eliminated.
2. Discussion of Related Art
From GB-A-2094927a method of the category corresponding to the preamble of the main claim is described. However, the solution is not enzymatic, and it has been found that the enzymatic solution which is used according to the invention exhibits a superior cleaning effect in comparison to the prior art method.
U.S. Pat. No. 4,936,994 describes a method of attacking and removing microbial slime in slime covered surfaces and maintaining a slime-free surface as in exposed cooling tower surface and in waste water treatment and paper making. This prior art method comprises the use of an effective amount of a composite enzyme preparation consisting of a ratio of 2 parts of cellulase, 1 part of .alpha.-amylase and 1 part of protease. In the first place, the method according to the invention is directed specifically to cleaning of heat exchangers after deposition of pulp particles, whereas the prior art method is directed to removal of microbial slime from surfaces of e.g. cooling towers. In the second place, the enzyme mixture used according to the invention is quite different from the enzyme mixture used in the prior art method.